Probably
The Evidence:
1. The Wii DOES have enough internal memory to hold character data when a character is about 30mb and the wii's internal memory is 500mb (I'm pretty sure) and if you didn't have enough space on your internal memory you could always use an SD card
2. Mewtwo and Roy both still have some character data on the Japanese disk, and have more memory than the other forbidden 7.
And just because mewtwo has a trophy does NOT mean he cant be a dlc character
Fox, Falco, Toon Link, Mario, Luigi, Peach, and Bowser also have multiple figures.
And yes, I want DLC, but I'm on the fence about its guarantee.
Other reasons I'd add to the "evidence" would probably be:
3. Sakurai said that there are more Assist Figures than there are Playable characters in his Iwata Asks interview, which was just one week before the game was released. This is only possible if more Assists are downloadable or added with a disc upgrade, etc.
Here's the exact quote:
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"The number of regular characters is impressive on its own, but the number of characters represented by the Assist Trophies is truly something."
"Yeah. I may have gone out of control. (laughs)"
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4. Many of the playable characters have "other" trophies. Could work the same way with Mewtwo or Dixie.
Heck, even on Melee some characters had double, even triple trophies.
5. The "Secret 7" all have just as much data on Brawl as the Alloys do.
I like that term "Secret 7". I think I'll coin it.
Who honestly wouldn't like more characters, even if it seems a little far-fetched...?